Current limiting means for a generator

ABSTRACT

The invention provides means of limiting maximum current conducted through windings of an electric machine having a rotor and a stator. By encouraging an appropriate leakage flux around a winding, a leak impedance can be achieved which may be used, according to the invention, to limit the maximum current in the winding as a matter of machine design.

TECHNICAL FIELD

The invention relates to a means of limiting the maximum current,including a short circuit current, in windings of an electric machinesuch as a generator or motor.

BACKGROUND OF THE ART

The need for preventing short circuit overloading of circuits inelectric generators and motors is well recognized. Protective externalcircuits and equipment are often provided with fusible materials orelectronic controls external to the windings of the electric machine,however internal short circuit conditions may occur within the windingsof a motor/generator that would not be detected or controlled byexternal fusing or controls.

One approach to providing protection within the windings is disclosed inthe inventor's U.S. Pat. No. 6,313,560. Due to the heat generated byhigh currents and high rotational speeds of machines used in “moreelectric” aircraft engines, the electric motor/generator and theinsulated wiring conductors must be sufficiently protected from theunlikely event of internal faults during operation, to ensure redundantsafety systems exist. Therefore, it is desirable to build in failsafemeans for controlling maximum machine current.

It is an object of the present invention to improve control over themaximum current passing through electric motor/generator windings,including limiting high short circuit currents.

Further objects of the invention will be apparent from the disclosure,drawings and description of the invention below.

DISCLOSURE OF THE INVENTION

The invention provides means of limiting a current, such as a shortcircuit current, conducted through windings of an electric machinehaving a rotor and a stator. Conventionally, the stator has a pluralityof slots, each slot having windings and having a slot gap adjacent tothe rotor and in communication with an annular air gap separating therotor from the stator. The invention uses a leakage flux phenomenon(which is typically considered in transformer design but not typicallyconsidered in machine design) to limit the current in an electricmachine. By controlling the leakage flux impedance, the maximum currentin a winding can be controlled by this phenomenon since the leakage fluxinduces a secondary voltage across in the winding which has oppositepolarity to the primary voltage, thereby permitting the current to becontrollably reduced/limited. For example, to encourage and controlleakage flux the designer may vary the gap width; axial length; radialheight; gap surface area; gap surface topography; and gap magnetic fluxpermeability.

The prior art teaches that flux leakage is inefficient, and to beavoided, reduced or compensated for wherever possible, to increasemachine efficiencies and reduce unwanted magnetic interactions, etc.See, for example, U.S. Pat. No. 5,955,809 and U.S. published patentapplication US2003/0042814. The present invention, therefore, departssignificantly from the prior art by encouraging a leakage flux, and doesso for the novel purpose of controlling a machine maximum current, forexample in the event of an internal short circuit.

IN one aspect, the invention provides an electric machine operable as analternator, the machine comprising: a moveable magnetic rotor assembly;and a stationary stator assembly mounted adjacent the rotor assembly,the stator assembly including at least one electrical winding disposedin at least one slot defined in the stator assembly, the at least oneelectrical winding being electrically connected to a machine outputadapted to deliver generated output electricity from the machine, theslot having an upper end, the slot upper end having a shape andconstruction, the slot upper end being disposed between the at least onewinding and the rotor, wherein, in use, movement of the rotor assemblyinduces a first alternating voltage and current in the at least onewinding, and wherein, said slot upper end shape and construction have anassociated inductance which is sufficiently high relative to saidinduced alternating current such that, in use, a magnetic flux flowsthrough said slot upper end and around the at least one winding inresponse to said induced alternating current in said winding, therebyinducing a second alternating voltage and current in the at least onewinding opposite in polarity to said first alternating voltage andcurrent, thereby limiting an output current in the at least one windingto a pre-selected value.

In another aspect, the invention provides an electric machine operableas an alternator, the machine comprising: at least one primary magneticcircuit, the primary magnetic circuit at least partially defined by astator assembly, a rotor and a plurality of permanent magnets mounted tothe rotor, the rotor mounted adjacent the stator assembly and adaptedfor movement relative to the stator assembly, the stator assembly havingat least one electrical winding disposed in at least one slot defined inthe stator assembly, the at least one electrical winding electricallyconnected to a machine output adapted to deliver generated outputelectricity from the machine, wherein, in use, relative movement betweenthe rotor and the stator assembly causes a primary magnetic flux to flowaround the primary magnetic circuit, the slot and winding being disposedin the stator assembly such that, in use, the primary magnetic fluxflowing around the primary magnetic circuit induces a primary voltageacross the winding and an associated alternating current flow in thewinding to thereby deliver output electricity from the machine; and atleast one secondary magnetic circuit defined in the stator assembly andencircling the winding within the stator assembly, the secondarymagnetic circuit at least partially defined by the slot, wherein thesecondary magnetic circuit has a magnetic inductance which issufficiently high such that, in use, said induced alternating current inthe winding induces a secondary magnetic flux of sufficient magnitude toflow around the secondary magnetic circuit to thereby induce a secondaryvoltage across the winding opposite in polarity to the primary voltageand of sufficient magnitude to limit a maximum current flow in thewinding to at least a desired maximum current flow limit.

In another aspect, the invention provides an electric machinecomprising: a stator, the stator having a plurality of spaced-apartteeth, a core portion, a plurality of slots defined at least in part bythe core portion and pairs of adjacent said teeth, the statoradditionally including at least one slot top portion extending between afirst pair of adjacent teeth to substantially close at least one of saidslots, the slot top portion having a shape and construction; a magneticrotor mounted adjacent the stator for movement relative to the stator;and at least one current-carrying winding disposed in said substantiallyclosed slot of the stator such that the at winding is encircled by saidfirst pair of adjacent teeth, the core portion and the slot top portion,wherein, when the machine is operated and the rotor is moved relative tothe stator to induce a current in the winding, a magnetic flux ofpre-selected strength is permitted to flow through the slot top portionby reason of the slot top portion shape and construction, therebycausing a maximum current in the at least one winding to be limited to apre-selected maximum magnitude.

In yet another aspect, the invention provides an alternator comprising:a stator assembly, the stator assembly having at least one electricalwinding disposed in the stator assembly, the at least one electricalwinding electrically connected to a machine output adapted to delivergenerated output electricity from the machine; a rotor having aplurality of permanent magnets mounted to the rotor, the rotor mountedadjacent the stator assembly and adapted for movement relative to thestator assembly, whereby in use relative movement between the rotor andthe stator assembly induces a primary voltage across the winding and anassociated alternating current flow in the winding; and at least onesecondary magnetic circuit circling the winding in the stator assembly,the secondary magnetic circuit having a selected magnetic inductance byreason of its shape and construction such that, in use, said inducedalternating current in the winding induces a secondary magnetic flux inthe secondary magnetic circuit sufficient to induce a secondary voltageacross the winding to thereby limit a maximum short circuit current ofthe at least one winding to a level below that at which irreparablethermal damage is caused to the machine.

In another aspect, the invention provides an electric machine operableas an electrical generator including a rotor assembly and a statorassembly, the stator assembly mounted adjacent the rotor assembly, thestator assembly including at least one conductor disposed in at leastone slot defined in the stator assembly, the conductor having a minimumwidth, the at least one conductor being electrically connected to amachine output adapted to deliver generated output electricity from themachine, the at least one slot having a opening defined in an end ofsaid slot, the end disposed between the at least one conductor and therotor assembly, wherein the slot opening is narrower than the conductorminimum width. In one aspect, the slot has a second slot opening on theside facing back iron which is sized to permit the winding to beinserted therethrough into the slot. In another aspect, the windings areinserted through an end of the slots. In both cases, this is donebecause the first slot opening has a size which is unsuitable forinserting the conductor through it into the slot.

Also provided by the present invention is a method of making a statorassembly for an electrical machine, the method including the steps ofproviding a stator having a first face and a second face, the first faceadapted to be positioned adjacent a rotor, the second face having afirst set of slots defined therein adapted to receive electricalwindings therein; inserting at least one electrical winding into thefirst set of slots; providing a back iron adapted to be mounted to thesecond face; mounting the back iron and stator to one another to providea stator assembly; and then providing a second set of slots in the firstface, the second set of slots communicating with at least some of thefirst set of slots. Another method includes the steps of providing astator having a plurality of internal slots therein, the stator having aslotless face adapted for facing an electric machine rotor, insertingelectrical windings into the plurality of internal slots, and thenproviding a plurality of openings in said slotless face, the pluralityof openings communicating with at least some of the plurality ofinternal slots having electrical windings therein.

Still other aspects of the invention will be apparent from the attacheddescription and figures. The invention is applicable to at least anelectric machine with an internal rotor and a surrounding externalstator, where the stator slots extend axially on an internal surface ofthe stator; and an electric machine with an internal stator and asurrounding external rotor, where the stator slots extend axially on anexternal surface of the stator. Application to other machines is alsotaught.

DESCRIPTION OF THE DRAWINGS

In order that the invention may be readily understood, embodiments ofthe invention are illustrated by way of example in the accompanyingdrawings.

FIG. 1 is an axial sectional perspective view of a stator and rotor,according to the invention, with axially extending slots housingconductor windings, and showing the axially extending slot gaps.

FIG. 2 is a detailed axial sectional view showing two stator slots withconductor windings and slot gap width dimension (G_(g)), also showingthe circulation of leakage flux with arrows.

FIG. 3 is a detail perspective view of a flux conducting surface of aslot gap of FIG. 2 indicating the axial length dimension (L_(g)) and theradial height dimension (H_(g)) of the slot gap.

FIG. 4 is a partial axial sectional view showing an alternative statorconstruction using a cylindrical internal back iron and T-shaped statormembers to define stator slots and slot gaps independent of the width ofconductors.

FIG. 5 is an enlarged cross-section of a rotor and stator according tothe prior art.

FIG. 6 is an enlarged cross-section similar to FIG. 5, however insteaddepicted is the device of FIG. 1 embodying the present invention.

FIG. 6 a is an enlarged portion of FIG. 6;

FIG. 7 is an enlarged cross-section of a rotor and stator according to afurther alternate embodiment of the present invention.

FIG. 8 is an enlarged isometric view of a stator according to yetfurther alternate embodiment of the present invention.

FIG. 9 is a graph representing the relationship between maximum current,leakage inductance and slot gap width, according to the presentinvention.

Further details of the invention and its advantages will be apparentfrom the detailed description included below.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention provides an apparatus and method for controlling and/orlimiting a primary circuit current, including a short circuit current,through the windings of an electric machine such as a motor, generatoror alternator, either permanent magnet or otherwise.

The inventor has discovered that a physical behaviour of theelectromagnetic circuit in a machine, known as the leakage flux, may beused to limit maximum currents in the windings of the machine. Leakageflux is well-known and understood in transformer design, but heretoforethe machine designer has typically only considered flux leakage in thepursuit of minimising such leakage to improve machine efficiency andprevent unwanted interactions (e.g. magnetic coupling) between permanentmagnet rotors and other subsystems.

In the present invention, however, leakage flux is encouraged as a meansto limit current inside the machine—as a matter of design. By creating aleakage flux which counters the primary flux path, the maximum currentin the machine may be limited. Leakage flux is encouraged through thechoice and selection of certain parameters of the machine'sconfiguration, as will be better described below.

In one embodiment, the invention is given effect through design controlof the stator slots. While it is indeed well-known to provide windingslots in a stator, the configuration and design steps taken to providesuch slots in the present invention are both believed to be novel. In asecond aspect, the stator slots are filled with a slot cap material.These and other specific embodiments of the present invention will nowbe discussed.

Referring first to FIG. 5, a prior art stator 50 and permanent magnetrotor 52 are shown schematically and, for ease of depiction only, areboth shown partially and as planar bodies. Rotor 52 has magnets 54arranged in and alternating ‘North’ and ‘South’ pole arrangement, and isseparated from stator 50 by a rotor air gap 56. Winding or windings 58are provided in slots 60 between adjacent teeth 62 in stator 50. Eachslot 60 has a winding gap 64 which permits windings 58 to be insertedinto slot 60, typically by an automated winding machine, as is wellknown in the art. The winding gap 64 is typically at least severalwinding-widths (or diameters) wide, to permit the winding to be providedaccurately and efficiently in each slot 60. In use, as rotor 52 passesover stator 50, a primary magnetic flux path 66 is set up, as magneticflux travels from rotor 52, across rotor air gap 56, down a particulartooth 62, around slots 60 and along the core of stator 50 and then backup a further successive tooth 62 to rotor 52, across rotor air gap 56.As air has a magnetic permeability of almost zero, and because thewinding gap 64 is quite large (relatively speaking), almost no magneticflux passes along the stator face (i.e. adjacent rotor air gap 56),because such flux is impeded by the winding gap 64. Magnetic flux alongsuch a “tooth-top” path is also intentionally impeded, by design, toensure the primary flux path usefully passes around windings 58.

Referring now to FIGS. 1-4, FIG. 1 illustrates an example of an internalstator 1 according to the present invention for placement inside anexternal rotor 10. In the embodiment shown, the stator 1 has a pluralityof stator slots 2 extending axially on an external cylindrical surfaceof the stator 1. Stator slots 2 have a slot gap 7 on a surface 8 to bedisposed adjacent rotor 10. As will be described in greater detailbelow, however, unlike the prior art the slot gaps 7 are not providedfor the purpose of inserting winding conductors 3 into stator and, infact, slot gaps 7 are preferably as narrow, and more preferably narrowerthan, the width of an individual winding conductor 3. For example, aslot gap 7 accordingly to the present invention may be only .040″ inwidth (depending on a machine's design), which will clearly beunderstood by the skilled reader to be too narrow to permit windings tobe inserted therein by prior art winding methods. This embodiment isthus clearly distinguished from prior art winding gaps, as will bediscussed in greater detail below. The invention is also applicable toan electric machine with an internal rotor and a surrounding externalstator and, in such a case, the stator slots 2 would of course extendaxially on an internal surface of the stator (ie. adjacent the rotor),as will be apparent from this description. The invention applies to amotor and a generator/alternator.

FIGS. 1 and 4 illustrate the general structure of a stator 1 accordingto the invention, having a cylindrical back iron 4 upon which aremounted axially extending T-shape teeth members 5 to define rectangularaxially extending stator slots 2 to house a plurality winding conductors3. Stator 1 is thus a composite stator composed in this case of backiron 4 and stator 5. The stator 1 has a plurality of stator slots 2where each slot 2 houses conductor windings 3. Each slot 2 has a slotgap 7 adjacent to the rotor 10 and in communication with an annularrotor air gap 20 (see FIG. 6) separating the rotor from the stator 2.

Referring now to FIG. 6, in use, as rotor 10 passes over stator 1, aprimary magnetic flux path 20 is set up, as magnetic flux travels fromrotor 10, across rotor air gap 22, down a particular tooth 5, aroundslots 2 and along back iron 4, and then back up a further successivetooth 5 back to rotor 10, across rotor air gap 22. However, unlike theprior art, in the present invention a secondary or leakage flux 6 flowis encouraged around each group of winding conductors 3 (in thisembodiment), by reason of the increased leakage inductance caused by thewidth of the winding gap 7 in relation to the magnitude of the currentpassing through winding 3. The secondary or leakage flux 6 isproportional to the current flowing through winding 3 and in a directionopposite to the primary flux 20. This phenomenon, as it is presentlyunderstood, will now be described in greater detail.

As indicated in FIGS. 2 and 4, when the machine is operated, the primarycurrent conducted through the windings 3 generates a magnetic leakageflux circuit 6 about the periphery of the stator slot 2 and passingacross the slot gap 7 as best shown in FIG. 4. It will be understood bythose skilled in the art that the high permeability of the materialsused to construct the back iron 4 and the T-shaped member 5 such assilicon iron, have a tendency to confine the magnetic circuit, (althoughas indicated in FIG. 4, a certain amount of magnetic flux fringing 9will occur in the peripheral edges of the slot gap 7). Materialspreferred by the inventor for construction are: samarium cobaltpermanent magnets, maraging steel (preferably 250 or 300) retentionsleeve, aluminum yoke, copper primary and secondary windings, siliconiron, SM2 or other soft magnetic material for the stator teeth andlaminated silicon steel for the back iron. The stator material is rigid.Slot 2 is sized sufficiently to house conductors 3. Preferably, slot gap7 is sized to provide a suitable leakage flux 6, as will now bedescribed.

Referring again to FIGS. 2 and 4, the leakage flux 6 circulation isindicated with arrows. FIGS. 2 and 3 show details of the slot gap 7parameters in a simple embodiment having an axially extending slot gap 7of width dimension G_(g) having an axial length dimension of L_(g) and aradial height dimension of H_(g). The gap surface area may be derived bymultiplying L_(g)×H_(g). However, it will be understood in light of thisdisclosure, that the gap geometry and surface topography may be variedconsiderably, for example as shown in FIG. 8, to include ridges, orother surface features to improve or otherwise affect the transmissionof magnetic flux or vary the distribution of magnetic flux extendingacross the slot gap 7. Further, the magnetic permeability and thereforethe gap flux density can be adjusted by the selection of materials anddimensions for the back iron 4 and T-shaped members 5. Alternately, aslot gap cap may be provided to cap slot gap 7 partially or entirely, asdescribed further below.

Optionally, the stator material including the back iron 4 and orT-shaped member 5 can be selected such that at least a portion of thestator has a Curie temperature which is below maximum design operatingbelow a temperature for the machine, such that (according to theteachings of the inventor's U.S. Pat. No. 6,313,560, the teachings ofwhich are fully incorporated into this disclosure by reference) themagnetic flux circulation through the stator material will be impededwhen the stator material acquires a temperature above the Curietemperature. Such design is preferably configured such that thesecondary or leakage flux 6 is less affected by the Curie-point ‘effect’than the primary flux path. It is preferably that the inventor'sCurie-point effect be maximized for primary flux flow and minimized forsecondary or leakage flux flow to gain the most satisfactory benefitfrom the use of such feature with the present invention. In any event,the present invention may be used in conjunction with one or more meansto thereby assist in providing maximum current protection to an electricmachine.

The multi-piece stator of FIGS. 1-4 and 6 may be provided in thefollowing steps: providing by any suitable means, a stator ring 5 havingslots 2, wherein slots 2 are on the side opposite rotor face 8;providing windings 3 into slots 2; mounting back iron 4 to thetooth-ring by any suitable method (e.g. by bonding); and then providingslot gaps 7 by any suitable method (e.g. cutting).

Referring to FIG. 6 a, the path 6 that the leakage flux travels aroundwinding 3 may be thought of as comprising several path components—inthis case components A-D. In the discussion above and below, the skilledreader will understand in light of this disclosure that, although theinventor prefers to focus design attention in applying the presentinvention to the portion of the stator denoted as path D in FIG. 6 a,one or more path components may be designed appropriately implement thepresent invention.

Referring to FIG. 7, a second embodiment of the present invention isshown. The reference numerals defined above will also be used to denotethe analogous features in this embodiment. In FIG. 7, stator 1 iscomposed of a single piece (i.e. back iron 4 and teeth 5 are integralwith one another) and winding 3 comprises a single conductor. Also inthis embodiment, a winding gap 30 may be partially or completely cappedor filled by one or more filler or slot cap members 32, preferablycomposed of a material having higher magnetic permeability than air, butless permeability than the stator material, and thus permitting asufficient leakage flux 6 to be induced, in use to permit the current inwindings 3 to be limited to a desired level, as was described withrespect to the previous embodiment. It will be understood, however, thatin this embodiment slot cap members 32 replace (preferably completely)winding gaps 7, in both space and function. The designer may select theslot cap and stator materials and dimensions according to the teachingsof this disclosure to manage the leakage inductance, and thus limit themaximum current of the machine, in a manner as will now be described inmore detail.

Referring now to FIGS. 1-4, the means by which the invention limits theprimary current conducted through the windings 3 of the stator 2 will bedescribed below.

The present method involves controlling the slot gap parameters toprovide a desired a leakage flux inductance and thus, leakage impedance.The leakage flux induces a secondary voltage in the windings 3 ofpolarity in opposition to the primary current, in accordance with“Lenz's law”, which dictates that an electric current induced by achanging magnetic field will flow such that it will create its ownmagnetic field that opposes the magnetic field that created it. In theprior art, however, the winding's “own magnetic field” was not permittedto flow in any appreciable way by reason of stator geometry (i.e. theair gap was too wide). The inventor has recognized, however, that if aninduced leakage flux is encouraged to encircle a winding, the effect ofthe opposing of magnetic fields on winding voltage can be usedadvantageously to limit the maximum possible current flowing through thewinding, thus providing the machine designer with a tool to providingintrinsic short circuit protection.

In the prior art, the winding gap (with air therein between) was solarge that very high magnetizing forces (B) would be required to force amagnetic flux to cross the gap. The magnetic permeability of air is, ofcourse, very low, being essentially that of free space (μ_(o)). At priorart winding gap distances, insufficient magnetomotive forces (mmf) wasprovided by the magnetic circuit to force a magnetic flux to cross thewinding gap. It is of course understood that, in prior art designs, asthe machine size grows, so too can the associated magnetizing forces,however winding size also increases to handle the increased currentload, and therefore so too does winding gap size. Thus, regardless ofmachine size, the magnetic permeability of the winding air gap remains abarrier to a magnetic flux path in the portion of stator (including theair gap) between the windings and the rotor.

It is generally known that the internal impedance of an electricgenerator/alternator governs the short circuit current of the machine.As is well understood, the internal impedance is related to the numberof winding turns, magnetic flux parameters, among other things, and iscommonly referred to as the “commutating inductance” or “commutatingimpedance” of the electric machine. Although a machine's commutatinginductance typically determines the machine's short circuit current, itis well-understood that this property cannot be effectively used by thedesigner to control the maximum current inside the electricalgenerator/alternator, since increasing the commutating inductance alsoincreases the voltage generated at a given speed. The short circuitcurrent is calculated by voltage÷commutating impedance, which bothincrease in proportion to speed (i.e. frequency). Therefore, increasingthe commutating inductance also increases the voltage generated at agiven speed and as such will not limit the short circuit current.

However, according to the present invention, the total impedance of theelectric machine can also be increased (to thereby limit maximumcurrent) by an additional phenomenon, namely the “leakage inductance” ofthe machine. The leakage inductance has an associated leakage impedancewhich is independent of the machine's commutating inductance impedance.Therefore, leakage inductance can be used to adjust the maximum currentwith little effect on the unloaded output voltage of the machine. Theleakage flux is a direct result of the current passing through thewindings 3 and, according to Lenz's law, is opposite in polarity. Thus,leakage flux can be used to control the short circuit current throughthe windings 3. The leakage inductance is proportionally to leakage fluxand thus varying the inductance permits the designer to achieve a fluxsufficient to attain the necessary voltage to limit the currentappropriately in the machine. The leakage inductance of a permanentmagnet alternator (or any machine) can be accurately defined andcontrolled by defining an appropriate shape or configuration for thestator, and by selecting appropriate materials for construction of thestator. For example, the designer may ensure the secondary magnetic pathhas sufficient inductance by varying the gap 7 parameters between thewinding slots 2. The leakage inductance (and hence the leakageimpedance) can also be adjusted in design, according to the designer'swishes by, for example, varying the width G_(g) of the slot gap 7 orarea of the gap 7 (i.e.: H_(g)×L_(g)) (in the case of FIGS. 1-4), or byvarying the permeability of the slot gap 7, i.e. by selecting a slot capmaterial and dimensions (in the case of FIG. 7). This, the inductance ofthe secondary magnetic circuit (i.e. the leakage path) can be selectedin view of the expected current in the winding, in use, to encourage asufficient leakage flux around the winding to induce the necessarycounter-voltage to limit the maximum current in the winding to a valueacceptable to the designer. Some iteration may be required in design.

It will be understood that complete elimination of the slot gap 7 is notpreferred unless a slot cap material has a lower magnetic permeabilityand/or impedance than the stator teeth-back iron, so as not todetrimentally affect the primary magnetic circuit. It will also beunderstood that in this specification, including the claims, the terms“shape”, “configuration” and “construction” are used to refer to statordesign parameters such as dimensions, relative proportions, materialselection (which may include the selection of “air” as a “material” inthe case of the selection of an air gap), magnetic permeability, and soon, which affect the amount of magnetic flux which travels around thewindings through the stator as the result of a current flow in conductor3.

According to the invention, the induced leakage flux which and isgenerated by the current flow in the windings and is encouraged (byreason of the stator geometry and material provided) to circulate aroundthe wire bundle of the windings 3. Therefore, altering the statorconfiguration to narrow (or close altogether) the gap between adjacentteeth tends to close the magnetic circuit around windings 3, thuscausing an increase in slot winding inductance without increasing thecommutating inductance of the machine. The total machine inductance isthus increased. The magnitude of the leakage flux 6 may be adjusted indesign, for example, by selecting gap dimensions (for example widthG_(g), and gap area A_(g), or height H_(g) length L_(g)) (in the case ofFIG. 1), or by selecting and sizing an appropriate slot cap material (asin the case of FIG. 7), as described above. The leakage flux 6, which isgenerated as a function of the current flowing in the windings 3 (i.e.,not as a function of the magnets on the rotor), results in a voltagebeing induced in the windings 3 which opposes the flow of current(inductive reactance) which induces the leakage flux 6.

The present invention teaches determining and selecting a magneticinductance of the portion of the stator immediately surrounding thewindings, including the air gap, if any, to thereby encourage asufficient magnetic flux to flow around the windings, to permit thedesigner to limit the winding maximum current by design. As windingcurrent increases, so too does the leakage flux and induced‘counter-voltage’ (which is subtractive relative to the primary currentin the winding), and therefore encouraging leakage flux has a limitingeffect on the output current in the winding. Increasing the overallleakage inductance (relative to the nominal maximum current in thewinding) of the secondary magnetic path encircling the winding therebypermits a sufficient leakage flux to be induced, which in turn inducesan opposite polarity voltage (according to Lenz's law) across thewindings of sufficient size to permit a desired maximum current limit tobe achieved. As mentioned, the inductive reactance voltage isproportional to the primary current flow through the windings.Therefore, as the primary induced current level tends to increase, sotoo does the counter balancing leakage-induced counter-voltage.Intrinsic means to limit the current passing through the windings isthereby provided by the invention. In effect, the invention involvesproviding a torriodal magnetic flux conductor of sorts around themachine windings. Preferably, a leakage inductance is selected in designsuch that the maximum winding current is low enough to negate thepossibility of thermal damage to the machine in the event of a shortcircuit, or other fault otherwise tending to increase the current in thewindings. In any event, the net result of the present invention is tolimit the current in the windings no matter what is causing the currentflow, be it an externally applied AC voltage or voltage induced by therotating magnetic field. Inductive reactance is directly proportional toa rate of change (or frequency) of the current. The counter-voltageinduced in by a permanent magnet (PM) machine winding 3 is directlyproportional to the frequency of rotation and, as such, this inventionadvantageously results in a constant short circuit limit valueregardless of the PM machine speed or unloaded output voltage value.

Leakage inductance is a function of the total flux around windings 3.Total leakage flux is a function of total flux density×area (i.e.φ=B×A), and is thus affected by the width of the air gap (or slot capwidth, as the case may be) and magnetic permeability of the gap (i.e.air or other material). Thus, in order to achieve a desired inductance,the designer may adjust, for example, the gap face area (i.e. L_(g)and/or H_(g)), the gap width (i.e. G_(g)) and/or the magneticpermeability of the gap (e.g. air, or by selected slot cap material) toachieve a desired leakage inductance. The designer may thus adjust theshape, configuration and construction of the stator assembly to achievethe disclosed current-limiting means. In general, however, other designconsideration may also affect the designer's choice of parameters. Forexample, to lessen the effects of ripple torque in the device, one maytend to choose a small gap width (i.e. G_(g)), and then size the otherparameters appropriately to achieve the intended design result limitinga maximum current in the manner intended. FIG. 9 plots the relativerelationship between slot gap width, leakage inductance and shortcircuit current for a given slot face area (L_(g)×H_(g)) and machineconfiguration. Demonstrated is the inverse relationship between leakageflux and short circuit (or maximum) current in the machine. Machinegeometry, etc. will of course affect the exact nature of the plottedrelationship.

In the prior art, the designer focused on manufacturing considerationsin selecting an electric machine's winding gap sizes, in an effort toensure that the selecting conductor sizes permitted the desired numberof turns to be efficiently provided in the stator slots using automaticwinding machines. Other manufacturing considerations also preoccupiedthe designer in selecting the winding gap size. In contrast, the firstembodiment of the present invention frees the designer from suchconsiderations by providing alternate means to provide the winding inthe slot, and therefore, the slot gap 7 on face 8 adjacent the rotor isavailable for application of the short circuit limiting concept of thepresent invention.

As discussed briefly above, in prior art electric machines (see FIG. 5),the size of the conductor windings 3 and other assembly parameterstypically define the width G_(g)of the gap 7. However, by the provisionof a multi-piece stator 1, thereby eliminating the need to insert thewindings 3 in the slot 7 (utilizing a back iron 4 which is assembledafter insertion of the windings 3), it is possible to set the slot gap 7parameters to any value desired to attain the appropriate leakageinductance, as described above. This may of course include selecting awidth G_(g) which is too narrow to permit the passage of the windings 3through it. As mentioned above, narrowing the gap width G_(g) closes themagnetic circuit around the individual slot windings 3.

Therefore, the necessary leakage flux impedance is determined andprovided during the design stage such that the short circuit current maybe limited by the designer using the present invention to achieve anacceptable maximum value, based on the thermal characteristics of themachine, and cooling scheme. Thus, the machine design itself may preventa temperature rise in a short circuit condition to a level that wouldcause thermal damage the insulation in the winding 3 and/or structure ofthe stator 1.

The method of the invention permits the designer to ensure that themaximum limited current in the winding 3 does not cause current highenough to melt down the windings or cause other critical damage. Theinvention may be used independently to limit the maximum availablecurrent and power deliverable by an alternator or generator design, ormay be used in conjunction with the inventor's Curie point protectionscheme, as described above. It is considered that an internal stator 1as illustrated in FIG. 1 is likely to be more easily fabricated byforming the slot gaps 7 on the external surface of the cylindricalmember. However, the invention will be understood as not being limitedto this type of stator 1 configuration. A two-piece stator is notnecessary to achieve the present invention in the first embodiment.Alternately, the windings may be directly inserted into stator throughthe ends of the slots (e.g. perhaps by threading or the use of amultiple-piece winding which is assemble on the stator).

It will be understood that in this description, and in the attachedclaims, the term “slot” is used to describe the portion of the statorreceiving the winding(s). A “slot”, therefore, may be a typical slot perse (i.e. having one open side along its length), or may be some otherrecess within the stator capable of receiving a winding(s) (e.g. seeFIG. 7, wherein the “slot” has no open sides along its length). The termslot is used, therefore, for convenience only, and it not intended tolimit the scope of the invention as described, or claimed.

Although the above description relates to a specific preferredembodiment as presently contemplated by the inventor, it will beunderstood that the invention in its broad aspect includes mechanicaland functional equivalents of the elements described herein. Forexample, any number of windings and any winding configuration may beprovided. The stator may be a single (integral) piece, or may bemulti-piece. The machine need not be a permanent magnet machine, andother types of machines may be advantageously adapted to incorporate thepresent invention according to the teachings of this disclosure. Theslot and tooth configuration may be varied to suit the machine's design.Skilled readers will recognize that still other modifications arepossible without departing from the scope of the inventions disclosedand claimed herein.

1. An electric machine operable as an alternator, the machinecomprising: a moveable magnetic rotor assembly; and a stationary statorassembly mounted adjacent the rotor assembly, the stator assemblyincluding at least one electrical winding disposed in at least one slotdefined in the stator assembly, the at least one electrical windingbeing electrically connected to a machine output adapted to delivergenerated output electricity from the machine, the slot having an upperend, the slot upper end having a shape and construction, the slot upperend being disposed between the at least one winding and the rotor,wherein, in use, movement of the rotor assembly induces a firstalternating voltage and current in the at least one winding, andwherein, said slot upper end shape and construction have an associatedinductance which is sufficiently high relative to said inducedalternating current such that, in use, a magnetic flux flows throughsaid slot upper end and around the at least one winding in response tosaid induced alternating current in said winding, thereby inducing asecond alternating voltage and current in the at least one windingopposite in polarity to said first alternating voltage and current,thereby limiting an output current in the at least one winding to apre-selected value.
 2. The electric machine of claim 1 wherein thepre-selected value is below a current magnitude at which the machinewill be thermally damaged by reason of such current magnitude passingthrough the winding.
 3. The electric machine of claim 1 wherein the slotupper end shape and construction includes an air gap defined in thestator assembly.
 4. The electric machine of claim 3 wherein the air gapis narrower than at least one electrical winding.
 5. The electricmachine of claim 1 wherein the slot upper end is completely closed. 6.The electric machine of claim 5 wherein the slot upper end shape andconstruction includes a portion composed of a material different than amaterial comprising a portion of the stator assembly defining the atleast one slot.
 7. The electric machine of claim 1 wherein the rotorincludes a plurality of permanent magnets.
 8. The electric machine ofclaim 1 wherein a portion of the stator assembly is comprised of amaterial having a Curie point below a threshold temperature, thethreshold temperature being a maximum temperature sustainable internallyin the machine without the machine suffering critical thermal damage. 9.An electric machine operable as an alternator, the machine comprising:at least one primary magnetic circuit, the primary magnetic circuit atleast partially defined by a stator assembly, a rotor and a plurality ofpermanent magnets mounted to the rotor, the rotor mounted adjacent thestator assembly and adapted for movement relative to the statorassembly, the stator assembly having at least one electrical windingdisposed in at least one slot defined in the stator assembly, the atleast one electrical winding electrically connected to a machine outputadapted to deliver generated output electricity from the machine,wherein, in use, relative movement between the rotor and the statorassembly causes a primary magnetic flux to flow around the primarymagnetic circuit, the slot and winding being disposed in the statorassembly such that, in use, the primary magnetic flux flowing around theprimary magnetic circuit induces a primary voltage across the windingand an associated alternating current flow in the winding to therebydeliver output electricity from the machine; and at least one secondarymagnetic circuit defined in the stator assembly and encircling thewinding within the stator assembly, the secondary magnetic circuit atleast partially defined by the slot, wherein the secondary magneticcircuit has a magnetic inductance which is sufficiently high such that,in use, said induced alternating current in the winding induces asecondary magnetic flux of sufficient magnitude to flow around thesecondary magnetic circuit to thereby induce a secondary voltage acrossthe winding opposite in polarity to the primary voltage and ofsufficient magnitude to limit a maximum current flow in the winding toat least a desired maximum current flow limit.
 10. The electric machineof claim 9 wherein the magnetic inductance of the secondary magneticcircuit is less than a magnetic inductance associated with the primarymagnetic circuit.
 11. The electric machine of claim 9 wherein thedesired maximum current flow limit is predetermined to limit thermalenergy generated in the winding at the desired maximum current flowlimit is insufficient to cause thermal damage to the machine.
 12. Theelectric machine of claim 9 wherein the magnetic inductance issufficiently high such that, in use, a short circuit current in thewinding is limited to a current magnitude below that at which thermaldamage is caused to the machine.
 13. The electric machine of claim 9wherein a portion of the secondary magnetic circuit passes between therotor and the winding.
 14. The electric machine of claim 9 wherein thesecondary magnetic circuit is defined at least partially by the slot andan air gap between a plurality of teeth of the stator assembly.
 15. Theelectric machine of claim 14 wherein the air gap has a width which isless than a minimum cross-sectional width of the winding.
 16. Theelectric machine of claim 14 wherein the air gap is disposed on thestator assembly between the winding and the rotor.
 17. The electricmachine of claim 9 wherein the secondary magnetic circuit is defined atleast partially by the at least one slot and a filler member.
 18. Theelectric machine of claim 17 wherein the filler member is disposedacross the slot.
 19. The electric machine of claim 17 wherein the fillermember is disposed between the winding and the rotor.
 20. The electricmachine of claim 17 wherein the filler member is comprised of a materialdifferent than a material comprising a remainder of the stator assembly.21. The electric machine of claim 9 wherein a portion of the primarymagnetic circuit is comprised of a material having a Curie point below athreshold temperature, the threshold temperature being a maximumtemperature sustainable internally in the machine without the machinesuffering thermal damage.
 22. The electric machine of claim 21 wherein amaterial in the secondary magnetic circuit has a higher Curie point thansaid portion of the primary magnetic circuit.
 23. An electric machinecomprising: a stator, the stator having a plurality of spaced-apartteeth, a core portion, a plurality of slots defined at least in part bythe core portion and pairs of adjacent said teeth, the statoradditionally including at least one slot top portion extending between afirst pair of adjacent teeth to substantially close at least one of saidslots, the slot top portion having a shape and construction; a magneticrotor mounted adjacent the stator for movement relative to the stator;and at least one current-carrying winding disposed in said substantiallyclosed slot of the stator such that the at winding is encircled by saidfirst pair of adjacent teeth, the core portion and the slot top portion,wherein, when the machine is operated and the rotor is moved relative tothe stator to induce a current in the winding, a magnetic flux ofpre-selected strength is permitted to flow through the slot top portionby reason of the slot top portion shape and construction, therebycausing a maximum current in the at least one winding to be limited to apre-selected maximum magnitude.
 24. The electric machine of claim 23wherein the pre-selected value is below a current magnitude at which themachine is damaged by reason of current of said magnitude passingthrough the winding.
 25. The electric machine of claim 23 wherein theslot top portion is disposed between the winding and the rotor.
 26. Theelectric machine of claim 23 wherein the slot top portion completelycloses the slot top.
 27. The electric machine of claim 23 wherein theslot top portion has an air gap defined therein.
 28. The electricmachine of claim 23 wherein the air gap is narrower than a minimum widthof the winding.
 29. The electric machine of claim 23 wherein the rotorincludes a plurality of permanent magnets mounted thereto.
 30. Theelectric machine of claim 23 wherein the plurality of spaced-apart teethand core portions comprises separate pieces of the stator assembly. 31.The electric machine of claim 23 wherein the plurality of spaced-apartteeth and core portions are integral with one another.
 32. The electricmachine of claim 23 wherein the plurality of spaced-apart teeth and theslot top portion are integral with one another.
 33. An alternatorcomprising: a stator assembly, the stator assembly having at least oneelectrical winding disposed in the stator assembly, the at least oneelectrical winding electrically connected to a machine output adapted todeliver generated output electricity from the machine; a rotor having aplurality of permanent magnets mounted to the rotor, the rotor mountedadjacent the stator assembly and adapted for movement relative to thestator assembly, whereby in use relative movement between the rotor andthe stator assembly induces a primary voltage across the winding and anassociated alternating current flow in the winding; and at least onesecondary magnetic circuit circling the winding in the stator assembly,the secondary magnetic circuit having a selected magnetic inductance byreason of its shape and construction such that, in use, said inducedalternating current in the winding induces a secondary magnetic flux inthe secondary magnetic circuit sufficient to induce a secondary voltageacross the winding to thereby limit a maximum short circuit current ofthe at least one winding to a level below that at which irreparablethermal damage is caused to the machine.
 34. The electric machine ofclaim 33 wherein a portion of the secondary magnetic circuit passesbetween the winding and the rotor.
 35. The electric machine of claim 33wherein the secondary magnetic circuit includes an air gap.
 36. Theelectric machine of claim 35 wherein the air gap is narrower than awidth of the winding.
 37. The electric machine of claim 35 wherein theair gap size is unsuitable to permit the winding to be insertedtherethrough into the stator assembly.
 38. The electric machine of claim34 wherein said portion is composed of a material having a lowermagnetic permeability than a remainder of the stator assembly.
 39. Theelectric machine of claim 33 wherein a portion of the stator assembly iscomprised of a material having a Curie point below a thresholdtemperature, the threshold temperature being a maximum temperaturesustainable internally in the machine without the machine sufferingthermal damage.
 40. The electric machine of claim 37 wherein a materialin the secondary magnetic circuit has a higher Curie point than saidportion of the stator assembly.